1. Field of the Invention
The invention relates to a method for determining distortion and/or image surface for an optical imaging system.
2. Description of the Related Art
In order to describe aberrations of optical imaging systems, use is currently being made of the Zernike coefficients (Z2 to Z37), for the definition of which reference may be made to the literature. It is not important here as to whether the literature employs somewhat different definitions, that is to say all these definitions are to be viewed as covered by the invention. The Zernike coefficients Z2 and Z3 characterize an aberration denoted as distortion which describes a wavefront tilt in the pupil plane of the optical imaging system referred to an x-direction or y-direction of an orthogonal xyz coordinate system whose z-direction is parallel to the optical axis of the imaging system. The Zernike coefficient Z4 characterizes an aberration denoted as image surface or defocus in the z-direction. Various methods are known for determining distortion and image surface.
In one of these known method types, a test pattern is imaged and evaluated by comparison, in order to determine an offset of the focal position relative to a nominal position. This offset depends in general on the illumination setting used when imaging and on the test pattern used. The distortion and the image surface are reconstructed from the focus offset measured values at several field points.
An optimization, following the measurement, of the imaging characteristics of the optical imaging system with reference to distortion and image surface is restricted in this pattern-dependent method type to the illumination setting used in imaging the test pattern. If other illumination settings are required in the imaging mode, the imaging system is possibly not in an optimally adapted state. A so-called MIS method has been proposed as remedy, in the case of which a plurality of test pattern measurements are carried out at different illumination settings and focus offset measured values dependent on the illumination setting are determined therefrom as described, for example, in Hans van der Laan et al., “Aerial image measurement methods for fast aberration set-up and illumination pupil verification”, Proc. SPIE 4346 (2001), pages 394-407. However, the accuracy of the determination of distortion and image surface with the aid of this method is limited.
A second, pattern-independent method type is based on the direct measurement of the wavefront aberrations or Zernike coefficients, for example, by means of shearing interferometry methods as described in the laid-open patent publications WO 0163233 A2 and DE 101 09 929 A1, or by means of a Shack-Hartmann method as specified in N. R. Farrar et al., “In-situ measurement of lens aberrations”, SPIE vol. 4000, page 18 (2000). In the case of such a determination of the distortion and/or the image surface, that is to say of Z⅔ and/or Z4, the accuracy of the wavefront measurement is mostly markedly dependent on the accuracy of the absolute calibration of the measurement system.